There are in my knowledge only three functions total in Flyspray where the SQL is not MSSQL Server compatible. Project::getDayActivityByProject, User::getDayActivityByUser and Backend::get_task_list. For the first two one, I have had an idea, it's even documented somewhere either here or Github, just can't find it just now (it was something about changing the grouping by date(from_unixtime()) on mysql, date(to_timestamp()) on PostgreSQL and tolerating one hour error when standard time/daylight savings time change happen, because the results were only used for drawing those two lines in Overview page, not very critical...).

But then the last one. It uses GREATEST. That does not exist in MSSQL Server. That one should be rethought. After doing that, it might even be as easy as to just add SQL server as a supported database to installer and start testing.